In the design and manufacture of aircraft, it is generally desirable to minimize the space required by components of the aircraft. One example of an approach to saving space in aircraft is the variety of proposals that have been made for shortening the landing gear when it is retracted into a stowed position in the aircraft. The shortening of the landing gear may be required by the initial design of an aircraft or may be desired in order to minimize design changes in future generations of an existing aircraft. There is currently a trend toward providing new generations of existing aircraft with longer fuselages. A longer fuselage generally requires longer landing gear to provide ground clearance for the tail on takeoff. When the landing gear in earlier generations has been retracted in its extended position, i.e. without contracting the landing gear, it is possible to minimize the design changes required by the lengthening of the fuselage by modifying the landing gear to shorten upon retraction so that it may be stowed within the existing wheel well.
The shortening of the landing gear shock strut upon retraction may sometimes be accomplished by providing an actuator which compresses the shock absorber spring. This approach has been used successfully on small aircraft. However, in larger aircraft the approach is impractical because of the relatively great retraction actuator effort required. A number of systems have been proposed which avoid the actuator effort problem by the use of mechanically or electrically operated hydraulic valves that operate to relieve the pressure in the shock absorber during retraction. Systems which rely on valves to relieve the pressure have the disadvantage of adding to the weight. and complexity of the landing gear and retraction system. In addition, the accumulators that receive the pressurized fluid to relieve the pressure in the shock absorbing mechanism may themselves require space, thereby decreasing the net space savings of the system.
Landing gear systems in which metered flow of a liquid acts on an air cushion to absorb shocks and which are shortened upon contraction are disclosed in U.S. Pat. Nos. 2,106,289, granted Jan. 25, 1938, to J. F. Wallace; 2,186,266, granted Jan. 9, 1940, to J. H. Onions; 2,294,918, granted Sept. 8, 1942, to R. L. Levy; 2,390,661, granted Dec. 11, 1945, to A. R. Parilla; 2,478,729, granted Aug. 9, 1949, to W. B. Westcott, Jr.; 2,554,581, granted May 29, 1951 to R. L. Levy; 2,621,004, granted Dec. 9, 1952, to B. N. Ashton et al.; 2,754,072, granted July 10, 1956, to S. Shapiro; and 4,291,850, granted Sept. 29, 1981, to W. Sharples. Such systems are also disclosed in French Patent No. 996,613, published Aug. 23, 1954, and granted to P. Lallemant; British Patent Specification No. 881,718 of the inventor G. S. Cranwell, published Nov. 8, 1961; and Swedish Patent No. 156,796, granted to Svenska Aeroplan AB, and published Oct. 30, 1956. A wholly pneumatic shock absorber is disclosed in Canadian Patent No. 448,266, granted May 4, 1948, to R. S. Sanford.
The Shapiro patent discloses aircraft landing gear having an outer cylinder, an inner cylinder, and a head portion. The head portion has a fixed length defined by two spaced piston portions which slidably engage the inner surfaces of the outer and inner cylinders, respectively. The cylinders and the head portion together define an annular chamber of fixed volume that surrounds an internal chamber. In its operational position for landing and taxiing, the position of the head portion relative to the outer cylinder is fixed. Impact loads during landing add taxiing cause the inner cylinder to telescope into the outer cylinder. A metering pin carried by the inner cylinder extends into the internal chamber defined by the head portion to compress an air spring formed within the head portion and absorb the impact loads. Upon retraction of the landing gear, linkage which is attached to the top of the head portion raises the head portion relative to the outer cylinder. The lower piston portion of the head portion engages stops carried by the inner cylinder to pull the inner cylinder upwardly and thereby shorten the overall length of the landing gear. This shortening of the landing gear during retraction is accomplished without changing the internal volume of any of the chambers in the shock absorber, except the small annular chamber between the lower portion of the outer cylinder and the upper portion of the inner cylinder. This annular chamber expands to assist in the retraction. The linkage which raises the head portion is fixed to the aircraft structure and holds the head portion in its fixed operative position during landing and taxiing.
The Cranwell British patent specification discloses landing gear with a relatively complicated arrangement of internal chambers and valves. On landing and taxiing, the landing gear strut contracts in two stages. The first stage of contraction has a relatively low resistance because one of the oil chambers is expanding and because flow resistances are relatively low. The second stage of contraction provides greater resistance for taxiing. In both stages, oil enters an upper chamber past a piston rod assembly to compress an air cushion formed in the upper chamber. On retraction of the landing gear, a hydraulic jack between the outer and inner cylinders telescopes the inner cylinder into the outer cylinder only to the end of the first stage of contraction. Therefore, there is relatively low resistance to the shortening of the landing gear on retraction.
The Swedish patent discloses landing gear in which metered flow into a pressure chamber of fixed volume compresses an air cushion in the pressure chamber to absorb landing and taxiing loads. During retraction of the landing gear, the inner cylinder is telescoped into the outer cylinder by a hydraulic cylinder extending between the inner and outer cylinders. In order to maintain the volume of the air cushion constant during retraction, hydraulic fluid from the inner cylinder is directed to an auxiliary chamber above the pressure chamber through a tube that extends upwardly through the pressure chamber. The top of the auxiliary chamber is defined by a movable piston that is raised during the retraction sequence to increase the volume of the auxiliary chamber without a pressure increase. The piston is moved upwardly by the pivoting of a linkage mechanism having one end attached to the aircraft structure and an opposite end attached to the piston. The pivoting of the linkage mechanism also operates an internal valve to open communication between the contracting chamber of the inner cylinder and the auxiliary cylinder.
A number of the other patents cited above also disclose landing gear in which a valve mechanism operates to allow fluid flow into an accumulator during the retraction procedure so that the gear can contract without raising the pressure in the shock absorbing mechanism. The accumulator may be positioned within or separately from the landing gear strut. Landing gear systems in which the pressure is relieved by bleeding air from the air cushion are disclosed by Wallace, Parilla, and Westcott, Jr. The Onions and the two Levy patents disclose systems in which hydraulic fluid is bled into a reservoir to allow the volume of a separate air cushion to remain constant during retraction. In the system disclosed by Sharpels, hydraulic fluid is drained from a separate chamber during retraction to allow a piston to move downwardly and thereby provide space for displaced fluid above the piston without increasing the shock absorber pressure. The Canadian patent to Sanford discloses a pneumatic shock absorber in which air is bled from the main chamber to relieve pressure during retraction.
Ashton et al. disclose landing gear which is contracted upon retraction with no pressure relief and which uses the energy stored by the compression of the shock absorber spring to help extend the landing gear. The French patent discloses a shock absorber which is contracted upon retraction with no apparent pressure relief.
U.S. Pat. Nos. 4,540,142, granted Sept. 10, 1985, to J. Veaux et al.; and 4,561,612, granted Dec. 31, 1985, to J. Masclet, each disclose landing gear which is contracted upon retraction but do not disclose any details of the shock absorbing mechanism or any means of pressure relief. There are also a number of patents which disclose systems for retracting landing gear struts and shortening the struts as they are retracted but do not disclose the structure of the shock absorbers associated with the struts. Retraction actuators and linkages are disclosed in U.S. Pat. Nos. 2,484,919, granted Oct. 18, 1949, to W. B. Westcott, Jr.; and 4,047,681, granted Sept. 13, 1977, to E. H. Hartel. Hartel does state that the strut is shortened against the force of the shock absorber spring. U.S. Pat. No. 2,567,114, granted Sept. 4, 1951, to C. E. Linn, discloses a latch mechanism for locking landing gear in a stowed position. West German Patent Document No. 1,756,287, published Apr. 9, 1970; and British Patent Specification No. 1,011,830, published Dec. 1, 1965, each disclose a lever system for raising a shock absorber mechanism within a landing gear strut to shorten the overall length of the strut during retraction. U.S. Pat. No. 4,630,788, granted Dec. 23, 1986, to J. Veaux et al., discloses helicopter landing gear in which hydraulic fluid is drained from separate chambers to allow the gear to contract or collapse and thereby move the helicopter into a "kneeling" position.
U.S. Pat. No. 2,735,634, granted Feb. 21, 1956, to J. P. Fosness, discloses an aircraft shock absorbing strut for landing gear which is stowed in an extended, rather than a retracted, position. The shock absorber includes a lower chamber into which pressurized fluid is fed to raise the aircraft nose for takeoff. The pressurized fluid first moves a piston which carries a metering pin upwardly toward a pressure chamber and then moves the inner cylinder of the shock absorber downwardly relative to the outer cylinder. This latter movement increases the length of the strut. U.S. Pat. No. 4,524,929, granted June 25, 1985, to D. F. Gebhard, discloses an aircraft landing gear shock strut in which the inner and outer cylinders are locked together in a compressed position prior to takeoff. A gas charge is added to the pressure chamber and then the inner and outer cylinders are released from their locked position. The gas charge provides a vertical force which jumps the aircraft into the air. After takeoff, the pressure is discharged from the chamber so that the shock absorber is ready for its shock absorbing function upon landing.
In the Parilla and the earlier Westcott patents, a linkage mechanism with one end attached to fixed aircraft structure operates to open a valve during retraction to bleed air from the air cushion. The Parilla system also includes a telescoping cylinder with one end attached to the bottom of the inner cylinder of the shock absorber and an opposite end attached to fixed aircraft structure. The latter attachment is offset from the pivot point of the outer cylinder of the shock absorber so that the telescoping cylinder automatically pulls the inner cylinder into the outer cylinder when the shock absorbing strut is retracted into a stowed position. The inner cylinder is similarly contracted by a cable in the earlier Levy patent and the Linn patent, by rigid struts in the Canadian patent, by a rigid link in the earlier Veaux patent, and by a linkage mechanism in the Ashton et al. patent.
The above patents and the prior art that is discussed and/or cited therein should be studied for the purpose of putting the present invention into proper perspective relative to the prior art.